1. Field of the Invention
This invention relates to transparent conductive thin films for use in electro-optical devices.
2. Description of the Related Art
Many electro-optical devices operating in the infrared (generally, wavelengths of ˜0.7 μm to ˜14 μm) require conductive thin films that are extremely transparent. Such devices include liquid crystal cells for beam steering, spatial light modulators, and optical switches for fiber optic communications, switchable and/or tunable polarization modification components, and long wavelength vertical cavity surface emitting lasers (VCSELs). Conventional transparent conductive oxides such as indium tin oxide (ITO), zinc oxide (ZnO), and tin oxide (SnO2) cannot fulfill the requirements of these applications. These materials are mostly transmissive in the visible; however, transmission in the infrared drops due to free carrier absorption. The transmission values versus wavelength for an ITO film on a glass substrate are shown in FIG. 4. For ITO films with a sheet resistance of about 140Ω/□ the transmission is about 98% at 1.3 μm and about 97% at 1.5 μm; 1.3 μm and 1.5 μm are critical values for fiber-optic communication and long wavelength VCSEL applications. Other possible optically transparent conductive materials such as doped semiconductor wafers or epitaxial layers (Si, Ge, GaAs, InP doped) have limited utility because they do not cover the entire spectral range, have a limited size, and are expensive.
U.S. Pat. No. 6,458,637, issued Oct. 1, 2002 to Jeffrey T. Cheung, directed to transparent and conductive zinc oxide films, describes the introduction of hydrogen or a hydrocarbon gas (in addition to oxygen) into the vacuum chamber during growth of the film using pulsed laser deposition from a 2.0 atomic wt. % gallium-doped ZnO target. This approach incorporated hydrogen atoms into the ZnO lattice during growth and activates them to behave as electron donors so that a low electrical resistivity film (0.9-3×10−4Ω-cm) could be grown at low temperature. The process was found to work only when gallium was also present in the lattice. The resultant films, with or without doping, had a transmissivity of no more then about 90% at wavelengths of 0.35 to 2.0 μm.
Cadmium oxide (CdO) films doped with indium (CdO:In) have been prepared in the past for flat-panel displays and solar cells which require high transparency to visible light. However, these films are well known for their toxicity, and therefore the prior art has steered away from developing such films. Minami et al. reports that CdO:In films have been prepared with a resistivity of the order of 10−5Ω-cm for flat-panel displays and solar cells, but he states that they are of no practical use because of Cd toxicity. (Minami, Tadatsugu, “New n-Type Transparent Conducting Oxides”, Transparent Conducting Oxides, Volume 25, No. 8, August 2000, p. 38). Further, because these films have a yellow color, which differentiates them from prior art transparent films that are clear in appearance, one skilled in the art would be steered away from using these films for any applications requiring transparent films.
Undoped CdO has been prepared by sputtering, MOCVD, and spray pyrolysis (Murthy, L. C. S. & Rao, K. S. R. K., “Thickness Dependent Electric Properties of CdO Thin Films Prepared by Spray Pyrolysis Method,” Bulletin of Material Science, Vol. 22, No. 6, pp. 953-957 (October 1999); Subramarnyam, T. K et al., “Preparation and Characterization of CdO Films Deposited by DC Magnetron Reactive Sputtering”, Materials Letter, Vol. 35, pp. 214-220, (May 1998); Baranov, A M et al., “Investigation of the Properties of CdO Films”, Tech. Phys. Ltr, 23, (10) pp. 805-806 (October 1997)). Transmission no greater then about 85% has been reported in the wavelength range of 0.6-1.6 μm.
Representative data, doping and method of fabrication for these references, along with additional references reporting on CdO films, are listed in Table 1 below.
TABLE 1LITERATURE REPORTING ON CDO FILMSThick.GrowthResistivityT % &ApproachDopantSubstrateÅTempOhm-cmrangePulsed LaserIntrinsic*GlassR. T. + post>10−360-90% @Sputtering1annealing0.5-2.0 μmActivated ReactiveIntrinsic*Glass350 4 × 10−470-80% @Evaporation20.5-.85 μmSolution Growth3Intrinsic*Glass2000R. T. + post2-5 × 10−475-85% @annealing0.5-0.9 μmIon beamIntrinsic*Glass500050-70 C. 5 × 10−370-80% @sputtering40.5-.85 μm**Spray pyrolysis4Intrinsic*Glass5000180-2252-5 × 10−370-80% @0.5-.85 μm**DC reactiveUndopedGlassR. T. + post102-10−340-85% @sputtering5annealing0.5-.90 μm6sputteringIndium 10−5Spray Pyrolysis7UndopedDC MagnetronUndopedGlass5-4985% @Reactive0.6-1.6 μmSputtering8,9MagnetronUndoped >7 × 10−460-70% @Sputtering100.5-0.9 μmSpray Pyrolysis11Fluorine80-90% @0.4-0.7 μm;<80% @>0.7-1.2 μmLow Press. CVD12UndopedGlass500-800 2 × 10−3*Intrinsic dopant means that donors in the materials are due to defects not impurities**Measured against glass slide as reference1Shaganov, II, et al., “Obtaining transparent oxide conducting coatings by pulsed laser sputtering” Sov. J. Opt. Technol. 48(5), p 280-282 (May, 1981)2Pahtak, Girish et al, “Deposition and properties of cadmium oxide films by activated reactive evaporation”, Thin Solid Films, 245, p 17-26 (1994)3Varkey, A J et al, “Transparent conducting cadmium oxide thin films prepared by a solution growth technique”, Thin Solid Films, 239, p 211-213 (1994)4Chu, T L et al, “Degenerate Cadmium Oxide films for electronic devices”, J. Electronic Materials , 19, p 1003-1005 (1990)5Tanaka, K et al “Electrical and optical properties of sputtered CdO films”, Japanese J. of Appl. Phys., 8(6), p 681-691 (June 1969)6Minami, Tadatsugu, “New n-Type Transparent Conducting Oxides”, Transparent Conducting Oxides, 25, (8), p 38-44 (August 2000)7Murthy, L. C. S. et al., “Thickness Dependent Electric Properties of CdO Thin Films Prepared by Spray Pyrolysis Method”, Bulletin of Material Science, 22, (6), pp 953-7 (Oct. 1999)8Subramarnyam, T. K et al, “Preparation and Characterization of CdO Films Deposited by DC Magnetron Reactive Sputtering”, Materials Letter, 35, pp 214-220, (May 1998)9Subramarnyam, T. K et al, “Influence of Oxygen Pressure on the Structural and Optical Properties of DC Magnetron Reactive Sputtered Cadmium Oxide”, Physica Scripta, 57, p 317-320 (1998)10Baranov, A M. et al, “Investigation of the Properties of CdO Films”, Tech. Phys. Ltr, 23, (10) pp 805-806 (October 1997)11Ferro, R et al “F-Doped CdO Thin Films Deposited by Spray Pyrolysis”, PMS. State. Sol. (a) 177, P 477-483 (2000)12Coutts, T J et al, “Search for improved transparent and conducting oxides: A fundamental investigation of CdO, Cd2SnO4, and Zn2SnO4”, J Vac. Sci. Tech., A18, (6), p 2646-2660, (November/December 2000)
There is a need for films with lower resistivity and low optical absorption at wavelengths that extend from the short-wavelength infrared (SWIR) (1-2 μm) through the mid-wavelength infrared (MWIR) (3-5 μm) and possibly into the long wavelength infrared (LWIR) (8-12 μm). As an example, devices operating in the fiber telecommunication bands at 1.3 or 1.5 μm require an optical transmission of at least 99%.